The present invention provides for new and useful nickel powders particularly for use as an electrode material such as in Base Metal Electrode Multilayered Ceramic Capacitors (BME-MLCC).
In the manufacture of BME-MLCC's generally suitable pastes of metal powders and pastes of dielectric ceramic powders are alternatively laminated to form a multilayered structure of alternative metal powder layers and ceramic powder layers, wherein after sintering the metal powders layers form internal electrodes with a dielectric between each two electrodes.
Such capacitor structures conventionally are known as Precious Metal Electrode Multilayered Capacitors (PME-MLCC), when palladium or palladium-silver alloy powders have been used as the metal powder.
According to recent developments attempts have been made to substitute the precious metals palladium and silver by less precious metals such as copper and nickel.
A number of problems have been associated with using these less precious metals, predominantly the insufficient oxidation resistance during sintering in the presence of organic materials present in the pastes for providing laminating properties, and the shrinkage of the metal powders during sintering due to their lower softening temperature as compared to the aforementioned precious metal or precious metal alloy. Particularly shrinkage and multicrystallinity of the metal powder material leads to delamination of the multilayered structure during sintering and cracks in both, the dielectric and the electrode layers. The insufficient oxidation resistance of the metal powders leads to uncontrolled oxygen uptake with the inclusion of impurities such as carbon unless sintering is made in a protecting atmosphere, such as argon containing a reducing gas such as hydrogen.
Present attempts to overcome or reduce these drawbacks aim to reduce the impurity level and to increase the crystallinity of Ni and Cu powders, particularly to increase the crystal grain size of the powders to close to the powder particle size. However oxygen uptake, shrinkage, and as a consequence delamination and crack formation despite these efforts are still inferior as compared to precious metal electrodes. Particularly, shrinkage of known nickel powders upon sintering is inversely proportional with the particle size of the powder. Accordingly, it was not possible to use powders of diameter smaller than about 0.2 μm diameter without producing large numbers of capacitors with delamination and crack defects. Accordingly, presently known nickel powders impose a serious limitation to the tendency of miniaturization of BME-MLCC technology, considering that the minimum electrode layer thickness is about 2 to 3 times the average powder particle diameter.